Electrophotographic material of zinc oxide treated with guanazole

ABSTRACT

ELECTROPHOTOGRAPHIC RECORDING MATERIAL CONTAINING PHOTOCONDUCTIVE ZINC OXIDE TREATED WITH GUANAZOLE AND PROCESSES OF ELECTROPHOTOGRAPHIC RECORDING USING SUCH MATERIALS. THESE MATERIALS ARE CHARACTERIZED BY LOW MEMORY EFFECT, IMPROVED CHARGING CHARACTERISTICS AND ARE CAPABLE OF ACCEPTING EITHER POSITIVE OR NEGATIVE CHARGES TO OBTAIN EITHER POSITIVE OR NEGATIVE COPIES.

nited States Patent 61 fice 3,692,522 Patented Sept. 19, 1972 3,692,522ELECTROPHOTOGRAPHIC MATERIAL OF ZINC OXIDE TREATED WITH GUANAZOLE RobertJoseph Noe, Mortsel, Jozef Frans Willems, Wilrijk, Albert Lucien Poot,Kontich, and Karel Eugeen Verhille, Mortsel, Belgium, assiguors toGevaert-Agfa N.V., Mortsel, Belgium No Drawing. Original applicationJune 6, 1969, Ser. No. 831,253, now Patent No. 3,634,080. Divided andthis application July 21, 1971, Ser. No. 164,821

Claims priority, application Great Britain, June 6, 1968,

27,045/68 Int. Cl. G03g 7/00 US. Cl. 961.8 23 Claims ABSTRACT OF THEDISCLOSURE Electrophotographic recording material containingphotoconductive zinc oxide treated with guanazole and processes ofelectrophotographic recording using such materials. These materials arecharacterized by low memory effect, improved charging characteristicsand are capable of accepting either positive or negative charges toobtain either positive or negative copies.

This invention is a divisional application of the United States Pat.application Ser. No. 831,253 filed June 6, 1969, now US. Pat. No.3,634,080, entitled Photoconductive Recording Material.

Photoconductive binder type coatings prepared by incorporating a finelydivided photoconductor in a binder are well known.

Commonly used are electrophotographic materials containing aphotoconductive layer comprising photoconductive zinc oxide dispersed inan insulating binder. Said layer is coated on a sufliciently conductivesupport, e.g. a clay-coated or glassine type paper base, from apaintlike coating mixture by conventional paper coating techniques.Since the zinc oxide has a brilliant white colour it forms an excellentbase for contrasting with the developer materials applied to it.Moreover zinc oxide is relatively less expensive and can be spectrallysensitized by a great variety of dyes. However, as already explained inthe United Kingdom patent specification 1,020,504 filed Dec. 29, 1961 byGevaert Photo-Producten N.V., zinc oxide possesses the less interestingproperty of having a relatively low dark-resistivity. Hence, in orderthat the electrical conductivity of a photoconductive recording layerbased on photoconductive zinc oxide shall be sufi'iciently small in thedark for retaining an electrostatic charge during the time needed forexposure and development, it is usual to incorporate in the recordinglayer some other substance with a higher electrical dark-resistivity.

For that purpose and by the need of a photoconductive recording layerwith sufliciently mechanical strength a binding agent is used improvingthe dark-resistivity of the recording layer.

According to said United Kingdom patent specification, zinc oxide istreated with certain organic phosphorus compounds, preferably organicoxyacids of phosphorus irnproving the dark-resistivity of thephotoconductive zinc oxide grains. By that treatment it is possible touse a larger number of binding agents and even such type of bindingagents that have no very low electrical conductivity. As a furtherresult of that treatment the photoconductive recording layer is lesssensitive to variations in relative humidity and may contain a muchhigher amount of photoconductive zinc oxide so that an optimumsensitivity is reached.

Unfortunately the introduction of said organic phosphorus compoundsgives rise to a rather large memory effect i.e. to a rather largeresidual conductivity in the time after exposure.

Quantitatively speaking the memory effect is expressed here by the ratioof the periods of time (t and t respectively) elapsing before byelectrostatic corona charging in the absence of light of the maximalcharge level is attained on a photoconductive recording layer that hasnot been exposed previously (time 2 respectively 90% of the maximalcharge level is attained on the same photoconductive recording layer(time t that immediately before its charging had received an exposure toactive light sufiicient to remove a charge that had been applied in aprevious step.

The present invention is based on the discovery that the treatment ofphotoconductive zinc oxide with a guanazole compound (treating agent)corresponding to the following structural formula (including its saltand tautomeric structures):

increases the dark-resistivity of a recording layer containing such zincoxide and gives rise to a small memory effect. The said treatmentimproves the chargeability of a photoconductive recording layercontaining photoconductive zinc oxide with a negative corona andmoreover besides the possibility of negative corona-charging offers theunexpected possibility of positively corona-charging the recording layerand obtaining thereon positive electrostatic charge images.

Thanks to the property of the recording material ac cording to thepresent invention of being capable to be charged positively as well asnegatively, it is possible with one and the same developer, e.g. anelectrophoretic developer with toner particles possessing a particularsign of charge, to reproduce one and the same original at will as apositive or negative copy depending on the sign of the electrostaticcharge applied to the recording layer of said recording material.

Thus, it is possible e.g. starting from a microfilm original to preparepositive as well as negative enlarged copies.

The same effects as described above are obtained when thephotoconductive zinc oxide is mixed with other photoconductivesubstances e.g. the photoconductive chalkogenides of cadmium moreparticularly photoconductive cadmium sulphide, crystalline mixed cadmiumsulphide selenide and cadmium selenide. The said chalkogenides may bedoped e.g. with zinc in order to increase their photosensitivity.

A small memory etfect of the recording material is advantageous in casesin which the recording material before its image-wise exposureaccidentally or not is overall-exposed to daylight or in which therecording mateiral has to be used in a multicolour reproduction system,wherein the recording layer is several times successively charged andexposed e.g. through halftone selection transparencies of themulticolour original to be re produced.

According to a practical method of determining the memory-effect therecording material to be tested is fixed onto a rotating disk whereon itis successively carried under a corona charging unit and under a probeor electrode, wherein a charge is induced being directly proportional tothe one present on the recording layer (ref. Cassiers, J. Phot. Sci., 10(1962) 58).

The time (t of reaching 90% of the maximal charge level is noted andthen the recording material after maximal charging is exposed tillcomplete discharging. Practically immediately thereafter charging isrepeated and again the time (t is noted for reaching 90% of the sec ndmaximal charge level which normally is not as high as the one obtainedin the first charging step. A usual rotating speed of the disk is 750r.p.rn.

The compound involved can be prepared according to the method describedin: J. Prakt. Chemie 88 (1913) p. 310.

(In order to achieve an optimum eifect, the treating agent is preferablycontacted in dissolved form with the photoconductive zinc oxide.

The present invention can be carried out in the manufacture of anelectrophotographic recording material, by allowing the zinc oxide tocome into contact with the treating agent at any stage of themanufacture. Thus, the contact can take place before or during thepreparation of a photoconductive coating composition, comprisingphotoconductive zinc oxide and a binder material, for forming aphotoconductive layer. Alternatively the contact can take place afterthe formation of the layer e.g. after coating a said compound onto asupport.

In order to obtain the desired effects it is not necessary for thetreating agent to contact the zinc oxide particles over the whole oftheir surfaces. Morevore, an electrophotographic recording materialaccording to the invention need not comprise only zinc oxide particleswhich have been treated in accordance with the invention. Thephotoconductive layer can incorporate a mixture of treated and untreatedzinc oxide grains. Some of the zinc oxide grains can be treated asdescribed in the United Kingdom patent specifications 1,020,504 filedDec. 29, 1961 and 1,020,505 filed Nov. 8, 1961 both by GcvaertPhoto-Producten N.V.

The following methods can successfully be applied for forming aphotoconductive layer composition according to the present invention.

(1) The treating agent is added to an aqueous dispersion of thephotoconductive zinc oxide. The treated zinc oxide is filtered off orcentrifuged, dried and then dispersed in a solution of binding agent.This method is especially suited for compounds that are soluble ordispersible in water.

(2) The photoconductive zinc oxide is dispersed in an organic solventwherein the treating agent is soluble or dispersible, whereupon thenecessary amount of treating agent is added. A binding agent can beapplied simultaneously or thereafter.

(3) The photoconductive zinc oxide, a binding agent and a solvent forthe latter are dispersed together e.g. by grinding in a ball-mill,according to the particular zinc oxide particle size desired. Thetreating agent is added before, during or after grinding.

(4) To a paper support a layer is applied from a composition containingthe treating agent and optionally a binding agent. Alternatively, thetreating agent is incorporated into the paper, e.g. during themanufacture of the paper. On top of this layer (or to the paper as thecase may be) a layer is applied containing untreated zinc oxide and abinding agent. During coating of the latter layer and/or during storageof the material some treating agent diifuses from the first layer or thepaper backing, to the photoconductive zinc oxide and is absorbedthereon. When this method is used and when a treating agent is employedhaving a marked antistatic character, the agent can be used in such anamount that a sufiicient quantity thereof remains in the paper to renderthe latter suiiiciently conductive for carrying off electrostaticcharges during exposure.

(5) To a paper backing is firstly applied a layer of untreatedphotoconductive zinc oxide, which is dispersed in a binding agent. Ontop of this layer a layer is applied containing the treating agent and abinding agent if desired. During coating of the second layer and/orduring storage of the material, some treating agent diffuses from thesecond layer to the zinc oxide in the first layer. The composition ofthe second layer can be selected so 4 as to impart required surfacecharacteristics to the recording material, e.g. to render the surfacesuitable for application of the developing processes described andclaimed in the United Kingdom patent specifications 1,020,503 and1,020,505 both filed Nov. 8, 1961 by Gevaert Photo- Producten N.V. It isalso possible to use in the second layer a binding agent selected sothat the adhesion of a developing powder thereto is either enhanced,thus enabling fixing to be improved, or decreased, thus permitting abetter image transfer.

The photoconductive zinc oxide is preferably prepared by the oxidationof zinc vapour, thus is preferably socalled French-type photoconductivezinc oxide.

The dark-resistivity of a photoconductive zinc oxide layer can beincreased to an optimum value by using increasing quantities of treatingagent. It the amount of treating agent, which gives an optimumdark-resisitivity, is exceeded, the latter decreases proportionally tothe amount of excess treating agent.

It has been established experimentally that an optimum dark-resistivityis obtained by adding to the composition of the photoconductive layerduring its manufacture, according to the activity of the selectedtreating agent (which in that case contains no cations, nor is appliedin complexed form) 0.01% to 10% by weight of such agent based on theweight of the photoconductive zinc oxide. Preferably 0.01 to 3% oftreating agent is used. The amount of treating agent required when thisagent is in salt or complex form is normally higher.

In order to obtain the same effect with corresponding salts or complexedcompounds, normally amounts beyond 0.5% by weight in respect of thephotoconductive zinc oxide are applied, e.g. between 0.5 and 40%.

It will be understood that if the treating agent is added to a layer orsheet adjacent to the photoconductive layer and allowed to contact thezinc oxide by diffusion, the required proportion of the treating agentin relation to the photoconductive zinc oxide generally is higher thanwhen the treating agent is incorporated into the zinc oxide layeritself.

When a photoconductive layer is formed comprising treated zinc oxide ina binder, the ratio of binding agent to photoconductor influence thequality of the photoconductive layer with respect to the photoconductiveproperties, mechanical strength and insulating power. It is preferred toemploy the binding agent(s) and photoconductor in a weight ratio of from1:3 to 1:9. If layers are used with a much larger proportion of bindingagent, the image sharpness tends to decrease. If layers are used withmuch lower content of binding agent than the minimum content specifiedabove, the mechanical strength of the coating may be isufiicient formany purposes.

The photoconductive recording material according to the presentinvention can be used in a wide variety of reproduction methods, andenables the production of very contrasty images that can be easily andconsistently obtained, even under widely difiering circumstances such asstrongly varying degrees of air humidity.

In combination with the treating agent used in the present inventionbinding agents possessing a higher specific dark-resistivity than thetreated zinc oxide as well as binding agents possessing a lower specificdark-resistivity can be used. Suitable binding agents are described inthe United Kingdom patent specifications 964,878 filed May 3, 1960 and1,020,054 filed Dec. 6, 1962 both by Gevaert Photo-Producten N.V.

The said treating agent is successfully used in combination with bindingagent(s) applied from an organic medium as well as in combination withbinding agents applied from an aqueous medium.

Binding agents and mixtures of binding agents suitable to be appliedfrom an aqueous medium are described in our published Dutch patentapplications 6608814 and 6608815 both filed June 24, 1966 and 6802733filed Feb. 27, 1968.

Binding agents and mixtures of binding agents applied from an organicmedium are e.g. described in the United Kingdom patent specifications1,020,503 filed Nov. 8, 1961 and 1,020,504 filed Dec. 29, 1961 both byGevaert Photo- Producten N.V.

Preferred binding agents contain acidic groups or groups from which anacid can be produced in situ. The content of acidic groups is, however,preferably not so high that the recording layer obtains a water-solublecharacter.

Very good results are obtained with vinyl copolymers that containorganophilic (hydrophobic) structural units and acidic structural unitsderived from a,B-ethylenically unsaturated carboxylic acid compounds,e.g. crotonic acid, acrylic acid, maleic aid, itaconic acid andmethacrylic acid compounds.

Homopolymers containing acidic structural units may be used in admixtudewith organophilic polymers. Polymers and copolymers from which acidicgroups can be produced in situ contain, e.g. carboxylic anhydride groupse.g. maleic anhydride groups and/or sulphonylchloride groups, which canbe hydrolized to sulphonic acid groups. The photoconductive zinc oxideis preferably dispersed in an organic medium (aromatic hydrocarbonsolvent(s), chlorinated hydrocarbon solvent(s) or lower aliphaticalcholo) by means of an acidic alkyl resin,copoly(ethylene/vinylsulphonyl chloride/vinyl chloride) or formaldehydeacetal of polyvinyl alcohol respectively.

Particularly suitable binding agents and binding agent mixtures forapplication in recording layers according to the present invention are:

( 1) (a) polyvinyl acetate (b) copoly(vinyl chloride/vinylacetate/maleic anhydride) (85/14/1% by Weight) (c) copoly(vinylchloride/vinyl acetate/vinyl alcohol) (91/3/6% by weight) (2) (a)copoly(ethylene/vinylsulphonyl chloride/vinyl chloride) (45.2/6.2/48.6%by weight) (b) alkyl resin (acid number:l2)

(3) poly(isobutyl methacrylate) (4) copoly(methyl methacrylate/isobutylmethacrylate) (20/80 by weight) (5) (a) poly(isobutyl methacrylate) (b)formaldehyde-acetal of polyvinyl alcohol (6) copoly(vinylacetate/crotonic acid) ,(94.6/5.4% by weight) (7) (a) copoly(vinylacetate-chrotonic acid) (93.4/6.6%

by Weight) (b) copoly(ethylene/vinylsulphonyl chloride/ vinyl chloride)(45.2/6.2/48.6% by weight) (8) (a) copoly(vinyl acetate/methyl acrylate/acrylic acid) (67/28/5% by weight) (b) copoly(ethylene/vinylsulphonylchloride/vinyl chloride) (45.2/6.2/48.6% by weight) (9) (a) styrolatedalkyd resin (b) copoly(vinyl chloride/vinyl acetate/ acrylic acid)(86.5/13.3/0.2 %by weight) (10) copoly(vinyl acetate/methylacrylate/acrylic acid) (87/12/1% by weight) (11) copoly(styrene/methylacrylate/ acrylic acid) (55/ 44.6/0.4% by weight) The treating agentapplied in the present invention does not exclude the increase ofphotosensitivity of the recording element, e.g. by spectralsensitization of the photoconductive zinc oxide.

Spectrally sensitizing dyes can be allowed to adsorb t0 thephotoconductive zinc oxide before, during and/or after thedark-resistivity increasing treatment. Spectral sensitizing agents whichcan be applied are described in the already mentioned United KingdomPatent specification 1,020,504 and in our published Dutch patentapplications 6717400 filed Dec. 20, 1967, 6805983 filed Apr. 26,

1968, 6704706 filed Apr. 3, 1967 and 6704768 filed Apr. 4, 1967. A verysuitable technique for applying the spectral sensitizing agents isdescribed in the published Dutch patent application 6704768 whichtechnique can also be applied successfully in respect of thedark-sensitivity increasing compounds used according to the presentinvention.

The photoconductive recording layers containing a treating agent forphotoconductive zinc oxide as described may contain in addition to thephotoconductive substance and binder any other type of dark-resistivityor sensitivity increasing compound e.g. the phosphorus compounds andother dark-resistivity increasing compounds described in the Belgianpatent specification 612,102 filed Dec. 29, 1961 by GevaertPhoto-Producten N.V., and additives known in coating techniques e.g.dispersing agents (see e.g. our published Dutch patent application6712156 filed Sept. 5, 1967), compounds influencing the gloss and/or thewear resistance of the coating, and compounds that counteract agingand/or oxidation of the layer, or that influence the thermal stability.When selecting any additives, preference is given to those which leastreduce the sensitivity and dark-resistivity of the photoconductivelayer.

The photoconductive composition treated according to the presentinvention may be coated on a support according to a known coatingtechnique, e.g. by spraying, whirling, dip-coating, or by a coatingtechnique wherein use is made of a doctor blade. The supports or basematerials are chosen in view of the particular charging, exposure,recording and/or transfer technique wherein the recording material isused.

In electrophotographic recording techniques, wherein the photoconductivelayer is electrostatically charged, the support has an electric volumeresistivity lower than that of the recording layer, preferably at least10 as low as that of the photoconductive recording layer. Suitablesupports are described, e.g. in the United Kingdom patent specifications995,491 filed Mar. 16, 1962, 1,020,503 filed Nov. 8, 1961 and 1,020,504filed Dec. 29, 1961 all by Gevaert Photo-Producten NV. and in the UnitedStates Patent No. 3,008, 825 of Warren G. Van Dorn and Osmar A. Ulbrich,Jr., issued Nov. 14, 1961. Preferably used supports are conductive papersupports, more preferably glassine type paper supports.

The photoconductive layer of an electrophotographic material which isprepared by starting from a coating composition according to the presentinvention, can be used for recording purposes, in which prior toexposure an electric charge is non-differentially applied according toknown methods. However, the material can also be used in recordingtechniques, in which the exposure step precedes the charging step. Forsuch a technique we may refer to, e.g., the United Kingdon patentspecifications 1,033,419 and 1,033,420 both filed Nov. 26, 1962 byGevaert Photo- Producten NV.

The charging as already explained can be effected with a negative aswell as with a positive corona.

A corona charging system suited for a rapid change from negative topositive corona charging operates with a so-called floating earth havingcorona wires of opposite charge sign under and in front of the recordinglayer to be charged.

Well established methods of developing electrostatic images includecascade-, powder cloud-, magnetic brushand fur brush-development. Thesemethods are based on the application of charged dry toner to the surfacebearing the electrostatic image. Other methods are based on the use ofliquids, either insulating (electrophoretic development is preferred) orconductive liquids (see e.g. the United States Pat. No. 2,907,674 ofKenneth A. Metcalfe and Robert J. Wright, issued Oct. 6, 1959 and theBelgian patent specifications 610,060 filed Nov. 8, 1961 and 625,335filed Nov. 27, 1962 both by Gevaert Photo-Producten N.V.). Developmentof a conductivity image based 7 on electrolysis is described egg. by J.A. Amick, RCA Rev., 20, 753 (1959).

The present invention is further illustrated by the following example.

EXAMPLE 360 g. of copoly(vinyl acetate/crotonic acid) (95.7/ 4.3% byweight) were dissolved in a mixture of 21.4 1. of dichloroethane and 1.6l. of ethanol. To the solution of the binder 12 kg. of photoconductivezinc oxide (French type) of an average particle size of 0.15,, and 8 1.of methyl isobutyl ketone were added. The obtained mixture washomogenized in a sand-mill, and to 43.280 kg. of the obtained dispersionwere added whilst stirring:

(CH2) r-S OzNH-COCH3 in dimethylformamide 45.5 ml. of a 1% by weightsolution of bromophenol blue in methanol.

The obtained dispersion was applied by knife coating to a conductivepaper base of 67 g./ sq. m. at a rate of 25 g. of solids per sq. m. Theobtained layer was dried at 80 C.

The dried recording layer was negatively coronacharged till 600 v. witha double corona, the corona wires in front of the recording layer andthose below the support having a potential difference of -5000 v. and+5000 v. respectively in respect of the ground.

The charged recording layer was projection-exposed for see. by means ofan incandescant tungsten filament lamp of 400 watt placed at a distanceof 40 cm. through a positive linewor k transparency and developed bymeans of an electrophoretic developer as described hereinafter.

A positive 7 enlarged reproduction of the positive transparency wasobtained.

The same recording layer was positively corona-charged till +500 v. withthe same double corona unit, of which the corona wires in front of therecording layer and those below the support had a potential differenceof +5000 v. and --5000 v. respectively in respect of the floating earth.

The positively charged recording layer was exposed in the same opticalenlarger as described above but to a negative microfilm images as anoriginal.

By using the developer as described hereinafter a 7X enlargedreproduction having reversed image values in respect of the negativemicrofilm image was obtained.

The developer was produced by diluting the concen trated developercomposition described hereinafter in a volume ratio of /1000 by means ofIsopar H (trade name for an isoparafiinic hydrocarbon mixture having aboiling range of 177-178 C., sold by Esso Belgium N.V., Antwerp,Belgium):

Carbon black (average particle size: nm.) g 30 Zinc monotridecylphosphate as dispersing agent g 1.5 Isopar H (trade name) ml 750 Resinsolution prepared as described hereinafter -g-.- 150 The resin bindersolution was prepared by heating 500 g. of Alkydal L 67 (trade name of'Farbenfabriken Bayer A.G., Leverkusen, W. Germany, for a linseedoil-modified 8 (67% by weight) alkyd resin and 500 m1. of white spiritcontaining 11% by weight of aromatic compounds at C. till a clearsolution was obtained and by subsequent cooling.

We claim:

1. A recording process wherein an information-wise pattern of increasedconductivity is produced in a photoconductive insulating recordingelement comprising photoconductive zinc oxide treated with a compoundcorresponding to the following structural formula and the salt form andtautomeric structures thereof:

by steps comprising information-wise exposing said recording element toactive electromagnetic radiation.

2. A method for recording and reproducing information comprising thestep of forming an information-wise electrostatic charge pattern byelectrostatically charging and information-wise exposing to activeelectromagnetic radiation an electrophotographic recording materialcontaining a photoconductive recording layer comprising photocondnctivezinc oxide treated with a compound corresponding with the followingstructural formula and the salt form and tautomeric structures thereof:

'3. A method for recording and reproducing information according toclaim 2, wherein the electrostatic charge pattern formed on said layeris made visible by development with electrostatically attractablematerial.

4. A method for recording and reproducing information according to claim2, wherein the recording layer has been applied to a conductive layer orsupport having a volume resistivity lower than that of the dark-adaptedrecording la er.

S. A method for recording and reproducing information according to claim4, wherein the support is a paper support.

6. A method for recording and reproducing information according to claim1, wherein the photoconductive zinc oxide is used in admixture with asubstance increasing its spectral sensitivity.

7. A method for recording and reproducing information according to claim2, wherein said compound is used in admixture with the photoconductivezinc oxide in an amount of 0.01 to 10% by weight in respect of thephotoconductive zinc oxide.

8. A method for recording and reproducing information according to claim1, wherein the photoconductive zinc oxide has been dispersed in a bindermedium containing at least one binding agent comprising acidic groups orgroups from which an acid can be produced in situ.

9. A method for recording and reproducing information according to claim8, wherein said binding agent is a vinyl copolymer containinghydrophobic structural units and acidic structural units derived froma,fi-ethylenically unsaturated carboxylic acid compounds.

10. A method for recording and reproducing information according toclaim 8, wherein the groups from which an acid can be produced in situare carboxylic anhydride groups or sulphonyl chloride groups.

11. A method for recording and reproducing information according toclaim 2, wherein the photoconductive zinc oxide has been used inadmixture with an organic oxyacid of phosphorus increasing thedark-resisitivity of the photoconductive zinc oxide.

12. A method for recording and reproducing information according toclaim 2, wherein the recording layer is electrostatically negativelycharged.

13. A method for recording and reproducing information according toclaim 2, wherein the recording layer is electrostatically positivelycharged.

14. A method for recording and reproducing information according toclaim 12, wherein after charging and information-wise exposure theformed electrostatic charge pattern is developed electrophoretically.

15. A method for recording and reproducing information according toclaim 13, wherein after charging and information-wise exposure theformed electrostatic charge pattern is developed electrophoretically.

16. A recording material containing photoconductive zinc oxide treatedwith a compound corresponding to the following structural formula(including its salt and tautomeric structures) compound corresponding tothe following structural formula (including its salt and tautomericstructures):

18. A recording material according to claim 16, wherein thephotoconductive zinc oxide has been dispersed in a binder medium in aratio by weight of from 1:3 to 1:9.

19. A recording material according to claim 16, wherein thephotoconductive zinc oxide has been dispersed in a. binder mediumcontaining at least one binding agent comprising acidic groups or groupsfrom which an acid can be produced in situ.

20. A recording material according to claim 19, wherein said bindingagent is a vinyl copolymer containing hydrophobic structural units andacidic structural units derived from a,B-ethylenically unsaturatedcarboxylic acid compounds.

21. A recording material according to claim 19, wherein the groups fromwhich an acid can be produced in situ are carboxylic anhydride groups orsulphonyl chloride group&

22. A reproducing material according to claim 16, wherein thephotoconductive zinc oxide has been used in admixture with an organicoxyacid of phosphorus.

23. A recording material according to claim 16, wherein thephotoconductive zinc oxide has been spectrally sensitized.

References Cited UNITED STATES PATENTS 2,218,077 10/1940 Zerweck et al.260-308 C 3,287,116 11/1966 Hoegl.

GEORGE F. LESMES, Primary Examiner M. B. WI'ITENBERG, Assistant ExaminerUS. Cl. X.R.

961 PC; 252-501; 260--308 R UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent N 3 I I Dated I Inventor-(s) Robert ph NOE et al It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 9, Claim 16, in the formula, correct to read as follows; H fi HN=NH I I --C=NE Column 10 Claim 22, line 25, change "reproducing" torecording Signed-and sealed this 8th day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents -'ORM PO-1050 (10-69) USCOMM-DC GOING-P69 E u.s GOVERNMENTPRINTING orncs: 1969 0-3se-sa4

